A direct-current power supply is equipped with a DC-DC converter, and thereby converts a battery voltage or a substantial direct voltage applied from a commercial AC power supply through a rectifier into a predetermined direct voltage, and then provides it to external loads that include another power converter or power system. The direct-current power supplies are installed, in particular, into apparatuses equipped with electronic circuits consisting of semiconductor devices, that is, electronic apparatuses, and provide the electronic apparatuses with constant direct voltages with stability.
For a battery-powered electronic apparatus, in particular, a mobile information apparatus such as a cellular phone, a notebook PC, a PDA, and a portable audio player, it is desirable to draw as much power as possible from the built-in batteries, that is, to improve the use efficiency of battery capacity since it leads the extension of the operable time of the electronic apparatus. In a direct-current power supply installed in a battery-powered electronic apparatus, for example, the DC-DC converter includes a step-up chopper and improves the use efficiency of battery capacity by its boost operation as follows. Here, the boost operation refers to an operation for maintaining a voltage conversion ratio (a ratio of the output voltage to the input voltage) higher than one, that is, raising the output voltage to the external load higher than the input voltage. The voltage conversion ratio higher than one is hereafter referred to as a boost ratio.
The battery voltage falls at a comparatively slow pace from the initial voltage over the period of the early-to-middle stages of the battery discharge. During the period, the battery voltage is maintained higher than the allowable lower limit of operating voltage in the battery-powered electronic apparatus with a proper setting of the type and cell number of built-in battery. The direct-current power supply maintains the step-up chopper in non-operation during the period, and provides the DC power supplied from the battery to the external load (that is, other devices in the electronic apparatus), for example, without being substantially converted. Thus, the operating voltage of the electronic apparatus is maintained higher than its allowable lower limit.
The battery voltage drops at a comparatively rapid pace in the last stages of the battery discharge. The direct-current power supply sets its desired voltage at, for example, a level higher than the allowable lower limit for the operating voltages of the electronic apparatus by a certain margin. The direct-current power supply starts the step-up chopper when the battery voltage falls substantially below the desired voltage. Thereby, the direct-current power supply raises the output voltage from the battery voltage to the desired voltage, and supplies it to the electronic apparatus. Thus, the direct-current power supply can maintain the output voltage applied to the electronic apparatus at the desired voltage, until the instant when the battery reaches the neighborhood of its complete discharge state. As a result, the electronic apparatus can use most of the battery capacity as its power.
The following is an example conventionally known as the above-described direct-current power supply installed in a battery-powered electronic apparatus. See, for example, Published Japanese patent application H05-137267 gazette. FIG. 19 is the circuit diagram of the conventional direct-current power supply 100. Input terminals 101A and 101B of the direct-current power supply 100 are connected to the positive and negative terminals of the battery B, respectively. Output terminals 102A and 102B of the direct-current power supply 100 are connected to the high and low side terminals of an external load L (that is, other devices in the electronic apparatus), respectively. Thereby, the direct-current power supply 100 converts the input voltage (or the battery voltage) Vi from the battery B into the output voltage Vo to the external load L.
The direct-current power supply 100 comprises a step-up chopper 103, a converter control section 104, a bypass switch 105, a bypass control section 106, and an input voltage detecting section 107. The step-up chopper 103 performs a boost operation by switching of the chopper switch 103S. The converter control section 104 detects the voltage Vo between the output terminals 102A and 102B, and compares it with the desired voltage to be supplied to the external load L. Here, the desired voltage ET is set, for example, to be higher than an allowable lower limit for the operating voltage of the external load L by a predetermined margin. The converter control section 104 further controls switching of the chopper switch 103S under a pulse-width modulation (PWM) scheme, based on the difference between the output voltage Vo and the desired voltage ET.
The bypass switch 105 is connected between the high side input terminal 101A and the high side output terminal 102A of the direct-current power supply 100, in parallel with the step-up chopper 103. When the bypass switch 105 stays in the ON state, the path from the high side input terminal 101A through the bypass switch 105 to the high side output terminal 102A acts as a bypass for the path inside the step-up chopper 103 that includes a series connection of an inductor 103L and a diode 103D. The bypass control section 106 controls switching of the bypass switch 105, and, in particular, maintains the bypass switch 105 in the ON and OFF state during the non-operating and operating periods of the step-up chopper 103, respectively, as described below.
The input voltage detecting section 107 detects the voltage between the input terminals 101A and 101B, that is, the battery voltage Vi, and compares the detected value with a predetermined threshold Ei, which is hereafter referred to as a start input voltage. Here, a voltage drop, which is hereafter referred to as a non-operating voltage drop, occurs between the high side input terminal 101A and the high side output terminal 102A during the ON period of the bypass switch 105, that is, the non-operating period of the step-up chopper 103. The start input voltage Ei is set, for example, to be higher than the desired voltage ET of the converter control section 104 by the upper limit of the non-operating voltage drop: Ei>ET.
The input voltage (or the battery voltage) Vi and the output voltage Vo of the direct-current power supply 100 change during the discharge period of the battery B, as shown in FIGS. 20A and 20B. Here, broken and solid lines show the change in time of the battery voltage Vi, that is, the discharge curve of the battery B and the change in time of the output voltage Vo, respectively. FIG. 20B is the enlarged view of the neighborhood of the point Ss (an agreement point between the battery voltage Vi and the start input voltage Ei) shown in FIG. 20A.
The discharge curve of the battery B is comparatively flat in the early-to-middle discharge stages of the battery B. Over this plateau, the battery voltage Vi is higher than the start input voltage Ei. The input voltage detecting section 107 informs the converter control section 104 and the bypass control section 106 of the battery voltage Vi higher than the start input voltage Ei. The converter control section 104 then maintains its non-operating state and maintains the chopper switch 103S in the OFF state. On the other hand, the bypass control section 106 maintains the bypass switch 105 in the ON state. Thus, during the period when the battery voltage Vi is maintained higher than the start input voltage Ei, that is, in the region I shown in FIGS. 20A and 20B, the step-up chopper 103 stops and the output voltage Vo is maintained lower than the battery voltage Vi by the non-operating voltage drop Von. Thereby, the output voltage Vo is maintained higher than the desired voltage ET.
At the non-operating period of the step-up chopper 103, that is, the region I shown in FIGS. 20A and 20B, the conduction losses are reduced with decrease of the non-operating voltage drop Von. The bypass switch 105 stays ON during the non-operating period of the step-up chopper 103 in the above-described direct-current power supply 100. Then, the current is divided into two branches between the high side input terminal 101A and the high side output terminal 102A: one of the branches flows through the series connection of the inductor 103L and the diode 103D inside the step-up chopper 103, and the other flows the bypass switch 105. Accordingly, the turning on of the bypass switch 105 reduces the resistance between the high side input terminal 101A and the high side output terminal 102A. Thus, the direct-current power supply 100 suppresses the non-operating voltage drop Von. As a result, the conduction losses of the direct-current power supply 100 during the non-operating period of the step-up chopper 103 are reduced below the conduction losses of direct-current power supplies with no bypasses. Therefore, the use efficiency of the battery capacity is maintained high.
The battery voltage Vi abruptly drops at the last stages of the discharge of the battery B. The input voltage detecting section 107 detects the drop of the battery voltage Vi to the start input voltage Ei. See the point Ss shown in FIGS. 20A and 20B. Then, the section informs the converter control section 104 and the bypass control section 106 of the detection. At that time, the converter control section 104 starts a PWM control. At the same time, the bypass control section 106 turns the bypass switch 105 off. Thereby, after the time Ts when the battery voltage Vi meets the start input voltage Ei, the step-up chopper 103 operates, raises the output voltage Vo higher than the battery voltage Vi, and maintains it substantially equal to the desired voltage ET during the period when the battery voltage Vi falls below the start input voltage Ei, that is, in the region II shown in FIGS. 20A and 20B. Thus, the direct-current power supply 100 can maintain the output voltage Vo at the desired voltage ET, until the instant when the battery B reaches its complete discharge state. As a result, most of the capacity of the battery B can be provided for the external load L as its power.
Lithium ion rechargeable batteries are extensively used for battery-powered electronic apparatuses, in particular, mobile information apparatuses. The lithium ion rechargeable batteries have advantages over other rechargeable batteries, in particular, in the high energy density. Recently, new-model lithium ion rechargeable batteries are developed. See, for example, Published Japanese patent application 2003-47238 gazette or EP1381135. New electrode materials are adopted in the new-model lithium ion rechargeable batteries. Thereby, the energy density is further higher.
In FIG. 21, a solid line shows the discharge curve of the new-model lithium ion rechargeable battery, which is hereafter referred to as a new-model battery, and a broken line shows the discharge curve of the current lithium ion rechargeable battery, which is hereafter referred to as a current battery. The discharge duration of the new-model battery is longer than the discharge duration of the current battery. In other words, the discharge capacity of the new-model battery is larger than the discharge capacity of the current battery. On the other hand, the new-model batteries have lower discharge end voltages and discharge curves with larger slopes, in comparison with the current battery. See the discharge end points E and E0 and the discharge curves PL and PL0 shown in FIG. 21.
The adoption of the new-model batteries as batteries for the mobile information apparatuses is desirable in view of the improvement in energy density. However, the adoption of the new-model batteries requires the changes of the circuit designs suitable for the low battery voltages. In particular, for circuits using as their operating voltages the battery voltages as they are, their circuit design must be changed so that their operations may be possible at lower voltages. Such design changes are not easy, in general. When the mobile information apparatuses have a communication function by radio waves, such as cellular phones, the mobile information apparatuses comprise wireless transmitter sections. Power amplifier sections included in the wireless transmitter sections usually use as their operating voltages battery voltages as they are, and amplify signals to be transmitted. Since the power amplifier section should provide a fixed power, its input current increases when the whole range of the change in battery voltage falls by the adoption of the new-model batteries. Accordingly, the efficiency of the amplification of signals decreases. Such decreased efficiency is undesirable since it obstructs the extension of the battery time. Therefore, the adoption of the new-model batteries requires high increase in efficiency under lower operating voltage conditions to the power amplifier section. It is tough to change in circuit design of the power amplifier section so as to satisfy such a requirement.
When the operating voltage of the battery-powered electronic apparatus is maintained on a conventional order, a battery voltage Vi of the current battery does not fall below the above-described desired voltage (for example, a value higher than an allowable lower limit of the operating voltage by a predetermined margin ET) until the last stages of the discharge. See a crossing point Ss0 between the broken line PL0 and the desired voltage ET shown in FIG. 21. On the other hand, a battery voltage Vi of the new-model battery may fall below the desired voltage ET earlier than the last stage of the discharge, that is, at the middle stage of the discharge. See a crossing point Ss between the solid line PL and the desired voltage ET shown in FIG. 21. Accordingly, the above-described direct-current power supply 100 is effective, in particular, in adopting the new-model batteries.
The conventional direct-current power supply synchronizes the start of the converter control section to the turning on of the bypass switch as described above. However, the converter control section can usually start its switching control after the expiration of a duration substantially longer than zero (which is hereafter referred to as a starting time) from the instant of the start. The starting time of the converter control section includes, for example, the initialization time of the converter control section, that is, the activating time of internal power sources for providing reference voltages, and the initialization time of latch circuits. Accordingly, the start of the switching control by the converter control section, that is, the actual start of the boost operation by the step-up chopper lags behind the turning off of the bypass switch by the above-described starting time, in the above-described conventional direct-current power supply.
The lag of the actual start of the boost operation by the step-up chopper from the turning off of the bypass switch brings the following problem in the conventional direct-current power supply 100, for example, shown in FIG. 19. In the region I shown in FIG. 20B, that is, before the time Ts when the battery voltage Vi meets the start input voltage Ei, the battery voltage Vi (shown by the broken line) is higher than the output voltage Vo (shown by the solid line) by the non-operating voltage drop Von. The bypass switch 105 is turned off and the converter control section 104 starts at the time Ts when the battery voltage Vi meets the start input voltage Ei. See the point Ss shown in FIG. 20B. However, the time Tf when the output voltage Vo begins to stay at a level equal to the desired voltage ET, lags behind the time Ts by the delay time ΔT. See the point Sf shown in FIG. 20B. Here, the delay time ΔT is a sum of the starting time of the converter control section 104 and the time (which is hereafter referred to as a recovery time) required for the output voltage Vo abruptly dropped in the starting time to return to the desired voltage ET by the boost operation of the step-up chopper 103. In the period from the time Ts to the time Tf=Ts+ΔT, the output voltage Vo drops abruptly and temporarily from the desired voltage ET, and produces an undershoot Us. The excessive undershoot Us of the output voltage Vo tends to suddenly abort the electronic device, that is, the external load L.
Accordingly, further improvement in reliability of the conventional direct-current power supply 100 requires the suppression of the occurrence of the excessive undershoot Us. The undershoot Us can be sufficiently reduced if, for example, the capacitance of the smoothing capacitor 103C is sufficiently increased. However, the increase in capacitance of the smoothing capacitor 103C scales up the whole of the direct-current power supply 100. The upsizing of the power supply section is undesirable since it obstructs the improvement in downsizing of electronic devices, in particular, such as mobile information apparatuses.
The battery voltage Vi is higher than the output voltage Vo during the non-operating period of the step-up chopper 103, that is, the ON period of the bypass switch 105. Conversely, the battery voltage Vi is lower than the output voltage Vo in general during the operating period of the step-up chopper 103, that is, the OFF period of the bypass switch 105. Accordingly, the bypass may include a diode instead of the bypass switch 105. At that time, the whole circuit can be scaled down since the bypass control section 106 is unnecessary. However, the forward voltage drop across a diode is, in general, larger than the ON-state voltage across a switching element. In other words, the conduction loss in a diode is higher than that in a switching element. Accordingly, the replacement of the bypass switch 105 with a diode has a disadvantage in the conversion efficiency.